The principle of manufacturing high purity hydrogen by using a hydrogen separation membrane is that a source gas (containing hydrogen) having a higher pressure than that of high purity hydrogen to be obtained is brought into contact with a hydrogen separation membrane using hydrogen permeable metallic foil to cause only hydrogen of the source gas to permeate, by which high purity hydrogen is obtained from the opposite side of the hydrogen separation membrane.
Conventionally, as a hydrogen separation unit used for manufacturing high purity hydrogen of this kind, there has been known a unit having a hydrogen separation membrane which consists of, for example, Pd or an alloy containing Pd and a porous support to which the hydrogen separation membrane is attached (for example, Japanese Patent Provisional Publication No. 62-121616 (No. 121616/1987), No. 5-76738 (No. 76738/1993), and No. 9-255306 (No. 255306/1997)).
In the above-described hydrogen separation unit, in order to get higher performance, that is, to enhance hydrogen separating performance, (A) to increase hydrogen permeation amount by decreasing the thickness of hydrogen separation membrane and (B) to increase a pressure difference between both sides of hydrogen separation membrane, that is, a pressure difference between source gas and hydrogen gas can be thought of.
With respect to item (A), for example, Japanese Patent Provisional Publication No. 62-121616 (No. 121616/1987) describes that a Pd-containing thin film (hydrogen separation membrane) is formed by vapor deposition such as CVD and PVD. Also, a method for obtaining a Pd-containing thin film by plating has been proposed (for example, Japanese Patent Provisional Publication No. 5-123548 (No. 123548/1993)).
However, in the case where a method for obtaining a hydrogen separation membrane by vapor deposition or plating is used, pores in the porous support must be closed by the deposition of a film, which necessarily increases the membrane thickness. If it is attempted to decrease the membrane thickness, a portion of porous support in which relatively large pores are formed cannot be closed completely, which may cause pinhole-shaped defects. Also, a rolled material sometimes contains inclusions with a diameter of several microns. Therefore, if it is attempted to decrease the membrane thickness only by rolling, there is a great danger of causing defects penetrating in the thickness direction.
On the other hand, the aforementioned Japanese Patent Provisional Publication No. 5-76738 (No. 76738/1993) describes that a Pd-containing thin film having a uniform thickness of 2 to 3 μm can be obtained by rolling.
However, in the case where a method for obtaining a uniformly thin hydrogen separation membrane by rolling is used, foreign matters with a diameter of several microns that exist between a roll and foil, which is a base material of hydrogen separation membrane, cannot be removed completely. Therefore, there is a fear that biting of foreign matters occurs at the rolling time and hence defect holes are produced in the hydrogen separation membrane.
In any of the above-described methods, the thickness of the obtained Pd-containing thin film is decreased uniformly. Therefore, in order to accommodate an increase in pressure difference in the above item (B), the construction of porous support must be complex, which poses problems in that the resistance to hydrogen permeation on the porous support side increases and that the manufacturing cost also increases.